There are conventional techniques disclosed in Patent Documents 1 and 2, for example.
FIG. 14 shows a cross-sectional structure of a general light emitting device employing an organic electroluminescence element (organic EL element), and how light propagates therethrough. An electrode 102, a light emitting layer 103, and a transparent electrode 104 are laminated in this stated order on a substrate 101, and a transparent substrate 105 is provided on the transparent electrode 104. When a voltage is applied between the electrode 102 and the transparent electrode 104, light is produced at a point S in the light emitting layer 103. The light is transmitted through the transparent electrode 104 directly or after being reflected by the electrode 102, enters a point P on a surface of the transparent substrate 105 at an angle θ from the surface normal to the surface, and is diffracted at this point and emitted into an air layer 106.
Let n′1 be the refractive index of the transparent substrate 105. When the incident angle θ exceeds a critical angle θc=sin−1(1/n′1), total reflection occurs. For example, light which enters a point Q on the surface of the transparent substrate 105 at an angle larger than or equal to θc is totally reflected and therefore is not emitted into the air layer 106.
FIGS. 16(a) and 16(b) are diagrams for describing the light extraction efficiency of the light emitting device, assuming that the transparent substrate 105 has a multilayer structure. In FIG. 16(a), let n′k be the refractive index of the light emitting layer 103, let no be the refractive index of the air layer 106, and let n′k-1, n′k-2, . . . , and n′1 be the refractive indices of a plurality of transparent layers provided between the light emitting layer 103 and the air layer 106 in order of distance from the light emitting layer 103, closet first. Let θ′k be the propagation direction (angle from the surface normal of a surface of refraction) of light emitted from the point S in the light emitting layer 103, and let θ′k-1, θ′k-2, . . . , θ′1, and θ0 be the angles of refraction of the surfaces of refraction in order of distance from the light emitting layer 103, closet first. In this case, according to Snell's law, the following expression is satisfied:n′k×sin θ′k=n′k-1×sin θ′k-1= . . . =n′1×sin θ′1=n0×sin θ0  (Expression 1)
Therefore, the following expression is satisfied:sin θ′k=sin θ0×n0/n′k  (Expression 2)
Eventually, Expression 2 represents Snell's law when the light emitting layer 103 directly contacts the air layer 106, regardless of the refractive indices of the transparent layers interposed therebetween, i.e., indicates that total reflection occurs when θ′k≧θc=sin−1(n0/n′k).
FIG. 16(b) schematically shows the range of light which can be extracted from the light emitting layer 103. The light which can be extracted is included within a pair of cones 107 and 107′ around the z axis as a center axis along with the surface normal of a surface of refraction, where their vertices are the light production point S, and their vertex angles are twice as large as the critical angle θc. If it is assumed that light beams emitted in all directions from the point S have the same intensity and that the transmittances at the surface of refraction are 100% at incident angles within the critical angle, an efficiency η at which light is extracted from the light emitting layer 103 is equal to the ratio of an area obtained as the intersections of a sphere 108 with the cones 107 and 107′ to the surface area of the sphere 108. The extraction efficiency η is given by:η=1−cos θc  (Expression 3)
Note that the actual extraction efficiency η is smaller than 1−cos θc since the transmittance within the critical angle is not equal to 100%. The total efficiency of the light emitting element is the product of the light emission efficiency of the light emitting layer and the extraction efficiency η.
Regarding the aforementioned mechanism, Patent Document 1 discloses an organic EL element in which the light extraction efficiency is improved by forming a diffraction grating on a substrate interface, an internal surface or a surface of reflection to change the incident angle of light to a light extraction surface, so as to suppress total reflection on a surface of a transparent substrate when light is emitted from the transparent substrate to the air.
Patent Document 2 discloses an organic EL element in which a plurality of transparent projections are formed on a surface of a transparent substrate so that light is prevented from being reflected on an interface between the transparent substrate and air, so as to provide a planar light emitting device having a satisfactory light extraction efficiency.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H11-283751    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-276581